Creating a drop down list with conditions

[Pauljj]

I want a department of managers to be able to select desk numbers for people to sit at but if someone is already sat there for that desks not to be available in the drop down. So a few examples of what I am trying to achieve below

Employee A sits at Desk A3 between 1500 and 1900
Employee B sits at Desk A1 between 0900 and 1800
Employee C sits at Desk B2 between 1200 and 1800


So if I enter in details for Employee D wanting to sit at desk B2 between 0900 and 1300 this should not show as Employee is sitting there for an hour but if employee D wants to sit there from 0900 to 1200 this is fine

I hope that sort of explains where I am coming from.....